Molten carbonate fuel cells comprise a plurality of fuel cell elements, each comprising an electrolyte layer sandwiched between an anode, layer and a cathode layer. The electrolyte layer is a porous layer soaked with molten carbonate. Charge transfer through the electrolyte layer from the cathode to the anode is done by carbonate ions.
Fuel gas which is reformed in the anode chamber or reformed fuel gas is fed to the anode chamber. If the fuel gas is methane, the following reforming reaction takes place:CH4+H2O→CO+3H2  (1)Carbon dioxide and oxygen are fed to the cathode chamber. The cathode reaction in a molten carbonate fuel cell is:4CO2+2O2+8e−→4CO32−;  (2)and the anode reactions are:3CO32−+3H2→3H2O+3CO2+6e−  (3)CO32−+CO→2CO2+2e−  (4)The overall reaction is:CH4+H2O+2O2→3H2O+CO2  (5)
In a conventional operation of a MCFC, the anode off-gas is recycled, typically after combustion of the non-utilized hydrogen and carbon monoxide, to the cathode chamber to provide for the carbon dioxide needed at the cathode layer. Air is fed to the cathode chamber to provide for the oxygen needed. The exhaust gas of the system, i.e. the cathode off-gas, comprises diluted carbon dioxide, usually in a concentration of about 3-5% (v/v).
In order to minimize the amount of carbon dioxide emitted to the atmosphere, it is advantageous to operate a MCFC in such a way that the carbon dioxide produced is obtained in a concentrated form. Carbon dioxide in a highly concentrated form, typically above 80% (v/v), can be efficiently liquefied and subsequently used in enhanced oil recovery or the recovery of coal bed methane. Also for effective sequestration of carbon dioxide, a concentrated carbon dioxide stream is needed. Carbon dioxide concentrated to about 50% (v/v), can usefully be applied in the food and paper industry.
In EP 418 864 A, a process is described wherein carbon dioxide containing combustion exhaust gas is fed to the cathode chamber of a MCFC. A high-concentration carbon dioxide gas is recovered from the anode off-gas. In the process of EP 418 864 A, there is still a diluted carbon-dioxide containing gas stream emitted to the atmosphere, i.e. the cathode off-gas. Moreover, the anode off-gas from which carbon dioxide is to be recovered is diluted with nitrogen.
Therefore, there is a need in the art for processes wherein the carbon dioxide in the exhaust stream is present in a high concentration, preferably at a concentration above 80 volume %.